The present invention relates to a vibration tool, for vibrating at a frequency between 10 kHz and 100 kHz, especially for applications in the welding, joining or cutting of materials, such as for example thermoplastic-based materials, in cleaning or emulsifying by the activation of a liquid medium, in gas spraying or else in shot blasting by blasting any kind of medium.
This invention relates more particularly to a large vibration tool, vibrating at ultrasonic frequency, more commonly called a sonotrode. The technical problem that has to be solved by the present invention is that of obtaining working areas having dimensions hitherto considered to be impossible to obtain by the use of the sonotrodes currently available on the market.
At the present time, when a person skilled in the art wishes to produce a sonotrode for the purpose of obtaining a surface of revolution such as a disc within the working region, it is impossible for the user to devise a circular sonotrode having a diameter greater than 120 mm with, as a condition, a constant amplitude value at every point on this working area. This is because the vibrations obtained during operation of such a sonotrode are tied on the internal stresses and on the appearance of additional vibration eigenmodes in longitudinal excitation, this being in a manner totally dependent on the increase in size of the sonotrode.
At the present time, the only means allowing a circular sonotrode of large diameter, that is to say a diameter possibly ranging up to 300 mm or more, to be envisaged consists in producing a hollow sonotrode, an illustrative example of which is shown schematically in FIG. 1 of the appended drawings. As this figure shows, this circular sonotrode is machined at its center over a diameter of at least twice the thickness of the wall 1 thus formed. In this way, a hollow 2 is obtained whose depth is limited for reasons of amplification and of transmission of movement at the corresponding longitudinal vibration node at the center of the length of the sonotrode. It should be noted that the length of the sonotrode thus produced is equal to a half-wavelength, which itself depends on the desired vibration frequency and on the speed of sound in the material used to machine this sonotrode.
Thus, as FIG. 1 shows, a sonotrode having approximately the shape of a bell is obtained and the frequency used remains an axial and non-radial vibration. This vibration remains at the limit of what can be used, since the deformation amplitude of the working area 3 is very far from being constant, since it varies by a factor of from 1 to 2 between the points lying on the circumference of the inside diameter and on that of the outside diameter defining this working area. Such a sonotrode therefore allows work only over a very, very narrow circular ring.
Moreover, owing to this deplorable distribution of amplitudes and because of the proximity of the many additional modes which impair the purity of the desired longitudinal vibration, this type of sonotrode is weak and is limited to extremely low working amplitudes, of the order of 30 xcexcm peak to peak for an element vibrating at 20 kHz. These characteristics are incompatible with the trend in the various applications at the present time and in the future, the requirements generally being for working amplitudes of the order of 50 xcexcm to 100 xcexcm for a frequency of 20 kHz.
To try to solve this problem, a number of constructional tricks have been envisaged, among which mention may be made, in particular, of longitudinal slots, which may or may not be emergent, as illustrates at 4 on the sonotrode shown schematically in FIG. 2 of the appended drawings. Such a solution makes it possible to eliminate the extraneous deformations associated with the additional eigenmodes in large sonotrodes, but contributes nothing to the desired quality of deformation over the working area.
Moreover, EP-A-0 313 425 discloses an axisymmetric sonotrode using the excitation principle based on a ring tuned to a defined frequency at so-called radial resonance, using a tube-shaped exciter resonating at this same frequency in a pure longitudinal mode. The longitudinal movement of the exciter is then converted into a transverse movement perpendicular to the axis of revolution of the sonotrode. The drawback with this known solution lies in the fact that the use of converting the movement of the exciter is a special case and does not allow, in particular, transmission of the vibration along one or more parallel axes.
Starting from this state of the art, the present invention sets the objective of producing a range of large sonotrodes, allowing working areas to be achieved which hitherto had been regarded as impossible.
Consequently, this invention relates to a large vibration tool, vibrating at ultrasonic frequency, especially for applications in the welding, joining or cutting of materials, especially thermoplastics, in cleaning or emulsifying by the activation of a liquid medium, in gas spraying or in shot blasting by blasting any kind of medium, made in the form of a circular sonotrode hollowed out at its center so as to produce a cavity allowing a shape approaching that of a bell to be obtained, characterized in that it comprises:
a linking mass providing surface coupling with a vibrational excitation element;
at least one circular membrane vibrationally excited at its center or at one end; and
at least one tube-shaped final element, the end of which defines the working area of the sonotrode, linked to the linking mass via the said membrane, the latter transmitting the vibration to the said final element whose length is defined by a longitudinal vibration mode tuned to the said excitation frequency.